1. Field of the Invention
The present invention generally relates to digital-to-analog converters (DACs). More specifically, a DAC using weights stored in a look up table is disclosed.
2. Description of Related Art
In a DAC, a binary input is converted to an analog voltage or current output that is proportional to the value of the binary input. For an n-bit binary input, the converter output is one of 2.sup.n possible analog outputs. For a DAC having voltage output, the voltages representing the possible outputs may be obtained with a network of capacitors, resistors, or current sources and current switches connected to the binary input.
Digital-to-analog conversion can be achieved using a number of different methods. One example is a ladder network using a network of resistors. A ladder network typically receives binary input voltages, each input voltage being 0 V or V.sub.reference, and provides an analog output voltage proportional to the value of the binary input. For example, a ladder network may have four stages for accepting four input voltages, representing four bits of digital data or sixteen possible input values, and an output voltage at one of sixteen possible output levels proportional to the value of the digital data input. Providing more stages in a ladder network allows more bits of binary input and requires greater voltage resolution. In general, the voltage resolution of a ladder network with n ladder stages is V.sub.reference /2.sup.n.
A DAC typically contains a set of weights generated by an analog circuit typically scaled in a binary fashion. The DAC receives a binary input which selects certain weights for the output. One problem associated with DACs is that the DAC presumes that the analog voltage or current circuit generates ideal output steps while the analog circuit generates non-ideal output steps. Thus, the DAC output voltage varies from the voltage selected by the binary input. The non-ideal characteristics of the output may be due to such factors as process variations and/or temperature.
What is needed is a DAC that eliminates the non-ideal characteristics of the output and/or compensates for irregularities in order to achieve a more accurate and more reliable digital-to-analog conversion. Ideally, the output of the DAC should be independent of variations in factors that are difficult to control such as the temperature and device characteristics that vary as a result of the manufacturing process. It is also desirable to provide a DAC that can be calibrated and recalibrated over time to take into account non-ideal characteristics of the output steps.